1. Field of the Invention
The present invention relates generally to a mobile communication system for transmitting packet data on an uplink. More particularly, the present invention relates to a method, in a Node B, of controlling uplink rates between primary User Equipments (UEs) for which a Node B managed cell is a serving Enhanced uplink Dedicated Channel (E-DCH) cell and non-primary UEs for which a different cell is a serving E-DCH cell.
2. Description of the Related Art
Asynchronous Wideband Code Division Multiple Access (WCDMA) communication systems use the E-DCH. The E-DCH was designed to improve the performance of packet transmission by introducing new techniques to uplink communications in the WCDMA communication systems.
The new techniques are those adapted for High Speed Downlink Packet Access (HSDPA), Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat Request (HARQ), and Node B-controlled scheduling.
FIG. 1 illustrates the basic principle of E-DCH transmission.
Referring to FIG. 1, reference numeral 100 denotes a Node B supporting E-DCH transmission and reference numerals 101 to 104 denote UEs that transmit E-DCHs. The Node B 100 evaluates the channel statuses and buffer occupancies of the UEs 101 to 104 and transmits scheduling grants to them based on the evaluation. The UEs 101 to 104 then determine their maximum allowed rates according to the scheduling grants and transmit data at or below the maximum allowed rates.
Since orthogonality is not kept among uplink signals from a plurality of UEs, the uplink signals interfere with one another. As more uplink signals are transmitted, interference with an uplink signal from a particular UE increases. The increase in interference with an uplink signal decreases the reception performance of the Node B. This problem can be solved by increasing the uplink transmit power of the UE. However, the increased transmit power interferes with other uplink signals which decreases the reception performance. Accordingly, the received power level of an uplink signal is limited to ensure reception performance. This can be explained with Rise over Thermal (RoT) defined asRoT=Io/NO  (1)where Io denotes the received total wideband power spectral density at the Node B, such as the total quantity of uplink signals received in the Node B, and No denotes the thermal power spectral density at the Node B. Therefore, a maximum allowed RoT represents radio resources available to the Node B for the E-DCH packet data service on the uplink. For example, a maximum allowed RoT represents radio resources such as a Received Total Wideband Power (RTWP) available to the Node B, for the E-DCH packet data service on the uplink.
FIG. 2 is a diagram illustrating a signal flow for a typical E-DCH transmission and reception procedure.
Referring to FIG. 2, reference numeral 202 denotes a UE that receives the E-DCH and reference numeral 201 denotes a serving Node B 201 for the UE 202.
In step 203, the E-DCH is established between the Node B 201 and the UE 202 by exchanging messages on dedicated transport channels. After the E-DCH setup, the UE 202 transmits scheduling information to the Node B 201 in step 204. The scheduling information includes the uplink transmit power or transmit power margin of the UE 202 from which uplink channel information can be derived, or the amount of transmission data buffered in the UE 202.
Upon receipt of scheduling information from a plurality of UEs, the Node B 202 performs Node B-controlled scheduling for the UEs based on the scheduling information in step 211.
When the Node B 201 decides to grant uplink packet transmission to the UE 202, the Node B 201 transmits scheduling assignment information to the UE 202 in step 205. In step 212, the UE 202 determines the Transport Format (TF) of the E-DCH based on the scheduling assignment information. In steps 206 and 207, the UE 202 transmits the TF information and the E-DCH to the Node B 201.
The Node B 201 checks errors in the TF information and the E-DCH in step 213. The Node B 201 transmits a Negative Acknowledgement (NACK) to the UE 202 on an Acknowledgement/Negative Acknowledgement (ACK/NACK) channel in step 208 if errors exist in either the Transport Format Combination (TFC) information or the E-DCH. If there are no errors in both the TFC information and the E-DCH, the Node B 201 transmits an ACK to the UE 202 on the ACK/NACK channel in step 208.
In the latter case, the UE 202 can transmit new information on the E-DCH since the E-DCH transmission is completed in step 207. In the former case, the UE 202 retransmits the same information on the E-DCH during a next Transmission Time Interval (TTI).
Node B-controlled scheduling is divided into two schemes: “rate scheduling” and “time and rate scheduling”. The rate scheduling increases or decreases a data rate for a UE, while the time and rate scheduling controls a transmission/reception timing as well as a data rate for a UE.
In the, rate scheduling scheme, the Node B increases, keeps, or decreases the data rates of all UEs requesting the E-DCH service by a predetermined level in every scheduling interval. In a system where a UE may have a TF set allowing the data rates of 16, 32, 128, 256, 384, and 568 kbps and the Node B allocates a data rate to the UE by indicating a one-level increase, keep, or decrease, if a current maximum allowed rate is 16 kbps and the Node B commands a rate increase during the next scheduling period, the one-level higher data rate from 16 kbps (32 kbps) becomes the maximum allowed data rate.
Since the rate scheduling scheme handles scheduling for many UEs, signaling overhead will be created if the amount of signaled information is very large. Therefore, the rate scheduling scheme uses a Relative Grant (RG) as scheduling information. The Node B sends signals of +1, 0, or −1 to the UE and the UE increases, keeps, or decreases its data rate by a predetermined level according to the received value.
Despite the benefit of less information and thus decreased signaling overhead on the downlink, the rate scheduling scheme takes a long time to rapidly increase data rate. Since the RG occupies one bit, RGs are signaled to UEs on a time-multiplexed common channel at UE-specific transmission timings or using UE-specific orthogonal codes.
The time and rate scheduling scheme additionally controls the E-DCH transmission timings of UEs. The time and rate scheduling scheme schedules part of many UEs and allows for a rapid rate increase or decrease. For this purpose, scheduling information is delivered by an Absolute Grant (AG). The AG carries a maximum rate to a UE and the UE sets its maximum allowed rate to the AG.
For example, if the UE now has a maximum allowed rate of 16 kbps and a large amount of data to be transmitted from the UE exists, the Node B can allocate 568 kbps to the UE in the next scheduling period so that the UE can transmit at up to 568 kbps. The Node B must have knowledge of a maximum available rate for the UE and the maximum available rate is determined by a TF set allocated to the UE. This is called a “Node B pointer”.
The time and rate scheduling requires a large amount of information to indicate an absolute rate. Therefore, when a dedicated channel is used for each UE, the transmit power of the downlink becomes high. In this context, the AG is delivered on a common channel such as a High Speed Shared Control Channel (HS-SCCH) in HSDPA and with a UE-identifier (UE-id) to indicate the UE for which the AG is destined.
The channel carrying the AG is called an Enhanced Shared Control Channel (E-SCCH). The uplink packet transmission system can reduce signaling overhead by fulfilling the delay requirements of UEs by supporting both the rate scheduling scheme and the time and rate scheduling scheme and thus using their advantages.
Now a description will be made of AG transmission on an Enhanced uplink Absolute Grant Channel (E-AGCH).
The E-AGCH is a common channel that carries an AG because every UE within a cell does not need to receive an AG during every TTI. A UE-id is allocated to the E-AGCH to identify a UE to be signaled. If the UE passes a cyclic redundancy check (CRC) using the UE-id, the UE transmits the E-DCH based on information received on the E-AGCH.
A description will be made of scheduling for Soft Handover (SHO) in a system supporting both the AG and RG.
An AG delivers a large amount of information with high power. The AG is decoded in a more complex way than the E-AGCH. Therefore, it is preferable that the UE receives an AG from one Node B. This one Node B is called a “primary Node B”. The UE selects a Node B that has the best downlink as a primary Node B. That is, the SHO UE receives an AG from the primary Node B and RGs from non-primary Node Bs other than the primary Node B.
Since a non-primary Node B is not authorized to schedule the UE, it does not transmit an RG indicating “up/down/keep” to the UE all the time. Instead, the non-primary Node B indicates a rate decrease if the ratio of RoTs from the other UEs in the SHO region is high. Otherwise, the non-primary Node B does not transmit signals so that the UE can operate based on the scheduling of the primary Node B. The indication is called an “overload indicator”. The overload indicator can be signaled to every UE on a dedicated channel, or on a common channel, considering downlink signaling overload.
FIG. 3 illustrates the uplink RoT of a cell in a typical SHO.
Referring to FIG. 3, the uplink RoT of a cell is the sum of an RoT 310 from noise always existent on a channel, an RoT 320 from legacy channels including DCHs and control channels, and RoTs 330, 340 and 350 from E-DCHs. Reference numeral 330 denotes an RoT from an E-DCH that can be transmitted without Node B-controlled scheduling, called “non-scheduled E-DCH”. Reference numerals 340 and 350 denote RoTs from E-DCHs requiring Node B-controlled scheduling, called “scheduled E-DCHs”. For example, the RoT 340 is from the E-DCHs transmitted by UEs for which the cell is a serving E-DCH cell, such as primary UEs. The RoT 350 is from the E-DCHs transmitted by UEs for a cell other than this cell which is a serving E-DCH, such as non-primary UEs. A serving E-DCH cell is defined as a cell that can transmit an AG to a UE. A cell is a serving E-DCH for its primary UEs and a non-serving cell for its non-primary UEs.
While not shown, the Node B sets a target RoT and performs scheduling such that a total RoT does not exceed the target RoT. Since the Node B cannot be directly involved in scheduling in relation to the RoTs 310, 320 and 330, the RoTs 310, 320 and 330 are not controllable by the Node B. A Node B scheduler of the Node B can manage the total RoT of the cell by controlling the RoTs 340 and 350. The RoT 340 from the primary UEs can be controlled by an AG or RG, and the RoT 350 from the non-primary UEs can be controlled by an overload indicator.
In the conventional SHO situation, it is not clear when a non-primary Node B has to transmit an overload indicator.
Accordingly, there is a need for an improved system and method for transmitting an overload indicator.